This invention relates to miniaturized circuits and more particularly to the interconnection of these circuits on different electrical devices, such as semiconductor devices, by means of a bond wire connection.
Small integrated circuit (IC) elements, dies, or chips, such as a radio frequency (RF) amplifier on a radio chip are sometimes connected to an off-chip device such as another semiconductor or a hybrid integrated circuit, for example an off-chip filter, by wire bonding. The wire bonding technique uses a wire bonding machine to fuse small wires to the contact points or bond pads of these small IC chips.
In hybrid IC chips, one or more chips or devices may be carried by a ceramic substrate. For example, high frequency such as radio frequency (RF) signals can be transmitted via a transmission line structure formed on one of the devices, such as on a surface acoustical wave (SAW) filter from another device, such as a chip containing radio circuitry. A transmission line is usually formed by any two conductors or wires, insulated from each other. In this case, the radio chip containing the on-chip radio circuitry may be smaller than the off-chip SAW device.
It is at the interface between the transmission line of the off-chip device and the small die that the bond wire connection is made. The transmission line of the off-chip device is fully capable of handling radio frequencies, but a crosstalk problem is encountered, for example, when attempting to connect the IC die to the off-chip transmission line.
When any two or more circuits are interconnected on different electrical devices having substrates, such as printed circuit boards, semiconductor integrated circuits (ICs) or other types of carriers, by a wire-bond transmission line, a certain amount of signal from one circuit may couple into an otherwise isolated circuit. The resulting induced signal is called crosstalk. This crosstalk situation can occur, when a signal, on one substrate, is transmitted to a subsequent circuit, on another substrate, across the wire-bond conductors. For example, in a radio transceiver, when a high frequency signal source, such as an antenna received radio frequency (RF) signal is routed from an RF amplifier in one substrate, to an RF filter, in another substrate, crosstalk can occur. In the other direction, during the coupling of a generated signal from a modulator, on the first or a third substrate, to a transmitter filter, in a fourth substrate, in the same radio, undesired crosstalk coupling can degrade the performance of the radio transceiver. Moreover, transmitter signals may undesirably couple into the receiver.
If one of the two conductors of the transmission line coupling the circuits is used as a grounding conductor, the circuit is referenced as single-ended and the transmission line is a single-ended transmission line.
Otherwise, two non-grounded conductors form a differential transmission line for a differential circuit. The geometry of the two conductors, and their relative positions, are set to establish a characteristic impedance to properly match the impedance from the first differential circuit to the subsequent differential circuit. A differential signal is applied across the two conductors by one differential generating circuit, and the signal travels down the transmission line to the differential receiving circuit, where the signal is measured as the difference between the two conductors. In other words, a differential circuit generates or receives a pair of complementary signals in a phase-inverted relation with each other, known together as a single differential signal.
The substitution of a differential wire bond transmission line for a wire bond single-ended transmission line can greatly reduce one type of crosstalk coupling called common-mode impedance coupling. Common-mode impedance coupling in a single-ended (non-differential) transmission line is caused by a non-zero parasitic impedance, generally called a ground-return resistance, that is unintentionally shared by two or more otherwise isolated circuits. This common impedance causes crosstalk. Therefore, by reducing the common or shared impedance of the ground conductor with a non-grounded conductor, common-mode impedance coupling is reduced.
In addition to common impedance coupling between circuits, there could also be capacitive (electric) and inductive (magnetic) coupling between the two conductors of the same transmission line if the two conductors are close together.
In the case of two adjacent transmission lines, differential transmission lines can also reduce the capacitive, or electric field, coupling and inductive, or magnetic field, coupling between the differential transmission lines, relative to single-ended transmission lines, if the distance between the two transmission lines is much larger than the separation between the conductors of one of the transmission lines. However, space is generally limited in these miniaturized applications, such as in a radio. Therefore, these differential transmission lines must be close together. This close situation increases the magnetic coupling and decreases the advantage of differential lines over single-ended lines.
A further reduction in crosstalk, over the plain differential line, for the two close conductors of the same differential line or for two close differential lines, can be achieved by twisting the two conductors to form a twisted-pair differential transmission line. This type of line is commonly implemented with two insulated round wires which are twisted about each other. This type of line can reduce crosstalk by reducing the inductive, or magnetic field, coupling. The crosstalk reduction is achieved by reducing the magnetic loop area of the line, and by changing the orientation of the magnetic field continuously over the length of the line.
An un-twisted pair defines a magnetic, or current, loop area over the entire length of the differential transmission line, with the wires being the long sides of a "rectangle". The area of such a loop defines the amount of current that can be induced in the wires by an external magnetic field, such as from an adjacent conductor.
By twisting the wires together, the loop area is minimized. The dimensions of the wire, and the number of twists per inch define the transmission lines characteristic impedance. Furthermore, the remaining loop is now twisted down the length of the transmission line, so that the normal to the magnetic loop area traces a spiral. When two such twisted pair lines are placed close together, the spiraling normals of the twists reduce the ability for the magnetic fields of one pair to induce a current in the other, such as from the modulator to the amplifier, if the circuits are closely spaced.
When interconnecting miniature circuits, such as printed circuits on a printed circuit board or a flexible circuit substrate, integrated circuits (ICs) on a semiconductor substrate, or hybrid circuits, the concept of twisted-pair differential transmission lines can be applied to reduce crosstalk. However, when connecting miniature circuits on two separate substrates with typical wire bonds, all the interconnections between substrates, even if the circuits are differential, are implemented by bond wires that are substantially parallel, and these parallel interconnections can lead to significant crosstalk.